1. Field of the Invention
The present invention relates to a control apparatus for a power train shifting mechanism.
2. Description of Related Art
For a four wheel drive vehicle in which all of the front and the rear wheels of the vehicle are driven, a center differential is disposed in its power train as disclosed in U.S. Pat. No. 4,804,061, thereby absorbing a difference between rotational velocities of the front and the rear wheels during cornering. The four wheel drive vehicles may be divided into a full-time four-wheel drive vehicle in which the four wheels are ordinarily driven and a part-time four-wheel drive vehicle in which two wheel and four wheel drive modes of operation can be manually shifted.
For the vehicles with part-time four-wheel drive as disclosed in Japanese Utility Model Publication (kokai) No. 127,232/1985, a shift mechanism movable in a stroke is provided such that a stroke position of the shift mechanism is changed to shift between two wheel and four wheel drive modes of operation. As disclosed therein, on the one hand, a power train is provided with a sleeve that is slidable in an axial direction and its displacement provides three positions, namely, a two wheel drive position at which transmission of a driving force to the front wheels or the rear wheels from one of the output elements of a center differential can be blocked, a four-wheel center-differential-locked drive position that blocks a differential operation of the center differential, and a four-wheel center-differential-free drive position that accepts a differential operation of the center differential. Accordingly, shifting such a power train requires at least three shift positions including the two wheel drive position, and the shift positions of the shift mechanism should take a position other than the stroke end position, namely, an intermediate position.
As shown in U.S. Pat. No. 4,846,010, on the other hand, the shift mechanism is driven by a motor to thereby reduce an operational force for shifting by the operator.
It is to be noted, however, that when the shift mechanism is driven by the motor, it may occur that the shift mechanism is not shifted to a given position for some reasons, whereby an overload could be applied to the motor. In order to prevent an overload to the motor, it is thus required that means for detecting a load, for detecting a magnitude of the load over the motor, be provided in such a manner that the motor is deenergized as the load detecting means is operated.
It has been found, however, that a situation may be likely to occur that an operator may misjudge an overload to deenergize the motor although no overload is applied to the motor, in such a manner that the operator selects the shift position when the overload is applied to the motor.
More specifically, the shift mechanism is usually constructed that, when it is shifted to one of the stroke end positions from an intermediate position other than the stroke end position, the driving, namely, the clockwise rotation, of the motor is deenergized upon a detection of the shifting to the stroke end position from the intermediate position. However, it may occur that the motor continues to be rotated in the clockwise direction due to inertia. At this time, notwithstanding deenergizing the motor, the motor may still continue rotating toward the stroke end position, thereby leading to operation of the load detecting means. As a result, it may become impossible to return the shift mechanism to the intermediate position from the stroke end position.